1. Field of the Invention
This invention is related in general to surfaces functionalized by vapor deposition and, in particular, to functionalization achieved by monomer deposition in the absence of monomer polymerization by radiation or other energy source.
2. Description of the Related Art
The term “functionalization” and related terminology are used in the art and herein to refer to the process of treating a material to alter its surface properties to meet specific requirements for a particular application. For example, the surface of a material may be treated to render it particularly hydrophobic and/or oleophobic and hydrophilic and/or oleophilic as may be desirable for a given use. Thus, surface functionalization has become common practice in the manufacture of many materials because it adds value to the end product. In order to achieve such different ultimate results, functionalization may be carried out in a variety of ways ranging from gaseous and wet chemistry to various vacuum deposition methods, sputtering, and plasma treatment.
Wet chemical processes have been used traditionally to treat with polymers and functionalize fibers that are otherwise inert or have limited surface functionality. These processes involve the immersion of the fibrous material in liquids or fluid foams designed to coat individual fibers and impart specific functionalities while retaining the material's porosity and ability to breathe. In spite of many claims it is clear that such wet-chemistry processes at best materially reduce the porosity of the substrate or, in the worst cases, essentially plug the interstices between fibers. Therefore, the functionalization of porous materials by wet-chemistry polymer deposition has produced the desired results in terms of surface functionality, but with the attendant deterioration of the mechanical characteristics of the underlying porous substrate.
Polymers applied by vacuum deposition have also been used successfully in the art to impart particular functional properties to films, foils and porous substrates without the limitations of wet coating processes. There is a large body of literature that addresses coatings using atmospheric and vacuum plasma processes (see for example U.S. Pat. Nos. 5,244,730, 5,302,420, 6,242,054, 6,397,458, 6,419,871, 6,444,274, 6,562,112, 6,562,690, 6,774,018, 7,244,292, 7,115,310, 7,255,291, 7,300,859 and 7,824,742). Vacuum plasma polymerization methods have been explored for at least 40 years. Plasma-based coating can be quite effective in coating and functionalizing porous surfaces, but that process has had little commercial success in applications such as web coating that require high speed treatment, mainly for two reasons. One is that the physical and chemical properties of these coatings are highly dependent on process parameters such as pressure, electrode geometry and type of applied voltage (DC, AC, HFAC, Microwave). Typically, a relatively long exposure to the plasma is required to assure that a high enough concentration of functional moiety is deposited on the surface. This leads to the second limitation, which is process time. Most methods cited in the literature require plasma exposure times in the order of seconds to minutes, which can be commercially acceptable for batch applications, but not for roll-to-roll applications that require functionalization of webs at speeds in the order of 100 to 1000 feet per minute, with coating times in the order of milliseconds, in order to create products that are both functionally and economically viable.
U.S. Pat. Nos. 4,954,371, 6,468,595, and 7,157,117 disclose high-speed vacuum deposition polymer coating processes that are free of these plasma polymerization limitations and have been used commercially to functionalize porous webs several meters wide at process speeds greater that 1000 ft/min. These processes utilize flash evaporation of a monomer material that condenses on a moving substrate, followed by radiation curing using electron beam or UV radiation. A variety of monomers, such as free-radical polymerizable acrylates, cationic polymerizable epoxies, vinyl monomers, and others, are used to functionalize a substrate surface with a wide range of functionalities that include hydrophobicity, oleophobicity, hydrophilicity, oleophilicity, antibacterial, color, anti-stain, metal chelating and antistatic properties. These processes are limited to the use of radiation polymerizable monomers that have high enough vapor pressure to be flash-evaporated but also low enough to allow condensation on the substrate. This limitation excludes many lower molecular-weight monomers that may be particularly desirable for specific applications.
The present invention was born out of a need to functionalize with monomer materials that are not easy to polymerize using radiation and/or that can be flash-evaporated but have poor condensation properties. Accordingly, the invention lies in a surface functionalization technology suitable for replacing the high speed in-vacuum radiation curing process in applications where it is necessary to use functional monomers that are difficult to condense and/or polymerize. Such monomers include, for example, perfluoro acrylates and methacrylates derived from various perfluoro alcohols that have been allowed for use by the U.S. Environmental Protection Agency in replacement of longer-chain fluorine-containing molecules that are easier to polymerize but have been categorized as hazardous materials. In addition, the invention relates to a process that is also suitable for implementation at high speeds, which is an absolute requirement for commercial viability.
This invention addresses the functionalization of web substrates processed at high speed in a roll-to-roll process; although it applies to all types of substrates, including 3-D objects, the main focus is on substrates that have a certain level of porosity. Textiles, non-woven products and paper substrates are fiber-based porous materials with inherent properties derived from the nature of the fibers. Synthetic and natural fibers (for example, polypropylene, nylon, polyethylene, polyester, cellulosic fibers, wool, silk, and other polymers and blends) can be shaped into different products with a great range of mechanical and physical properties for applications that include protective uniforms, biomedical fabrics and membranes, housing products, and filter media for gas and liquid filtration. The porosity of these materials usually serves a necessary function, such as gas and/or liquid permeation, particulate filtration, liquid absorption, etc. Therefore, any subsequent treatment designed to further modify the chemical properties of the fibers by appropriately functionalizing them must be carried out, to the extent possible, without affecting the porosity of the material.